1. Different currency
Test in each market you sell to. If the number needs to differ by market, that needs a rule per market — see Advanced rules.
2. Different products in the cart
Product and collection limits only apply when the cart contains those items. Your test cart almost certainly had them; theirs may not have, or may have had items from an overlapping collection carrying its own limit.
3. Overlapping collection limits
If a product sits in two collections that both carry limits, both apply. It is possible to build a combination a customer cannot satisfy — they remove items to fix one limit and trip the other.
Map your collection overlaps if customers report being stuck.
4. Quantity versus amount
A cart of six cheap items passes a quantity minimum and fails an amount minimum. If reports are inconsistent across different baskets, check which of the two you actually set.
5. Language, for the message rather than the block
If the block is right but the wording is wrong, that is multilingual fallback — an untranslated language gets the default. See multilingual messages.
Reproducing it
Ask for: the country, the currency shown at checkout, and roughly what was in the cart. Rebuild that in a private window.
Note that device does not matter for checkout blocking — it runs on Shopify’s servers. Device only affects the storefront quantity lock.